The remediation of arsenic from contaminated surface waters is a problem worldwide (Mandal, 2002). In terms of acute toxicity of inorganic arsenic; As(III) is more toxic than As(V). Either arsenate or arsenite can be the dominant form in aqueous systems depending on the reduction potential of the environment. Arsenate generally is the dominant form in oxic waters while arsenite dominates in sulfidic and anaerobic waters including deeply circulating geothermal waters. In strongly sulfidic environments that are near saturation with respect to a naturally occurring arsenic mineral, orpiment (As2S3), arsenic sulfide complexes may form with a generic formula HnAs3S6(n−3) (Li, 1995).
The toxicity of arsenic to humans at low concentrations has led to increased interest in arsenic remediation chemistry (Li, 1995). Drinking water standards for arsenic in many countries (USA and Europe) have been lowered to 10 ppb. Past research for remediation of arsenic contaminated water sources has focused on removal of arsenic by adsorption (Kaltreider, 2003; Ghurye, 1999; Balaji, 2002; 2005) or co-precipitation processes (the so-called ferrihydrite process). Initially, addition of ferric chloride to water to produce a precipitate of ferric hydroxide was a favored method of arsenic remediation (Cheng, 1994), and later this method was replaced by the use of a fixed bed of iron filings (McNeill, 1995). Both of these methods are often referred to as the “ferrihydrite process”. More recently using fixed beds of zirconium, titanium and (to a lesser degree) aluminum oxides are by far the most favored approach for drinking water and there are thousands of these systems operating successfully in the USA ranging from systems designed to treat water at a single faucet to systems treating thousands of gallons of water per minute (Sun, 2006; Manna, 2004). However, all of these technologies have the significant disadvantage of creating a large amount of sludge for ultimate disposal; also the arsenic cannot be recovered and there is the possibility of leakage/leaching of colloidal oxides into the aquifer (Rosengrant, 1990; Manning 1995; Ghosh, 2004; Shaw, 2008; Meng, 2001). Other methods suffer from a lack of specificity, low selectivity over sulfate (ion exchange), low mass-to-volume concentrations (bio-reduction), and/or high cost (membrane technologies).
Recently, anion exchange resins based on polystyrene and containing quaternized ammonium sites doped with iron have become available which claim significant arsenic selectivity over sulfate (ASM-10-HP, Resintech Inc., West Berlin, N.J.; Moller, 2008; Sarkav, 2007; Munoz, 2008; Rau, 2003). Polystyrene resins modified with oxoanion chelating ligands have also shown promise but have yet to be commercialized (Streat, 1986; Fish, 1985). More recently polystyrene modified with poly-dentate ligands and Zr(IV) has shown selectivity for arsenate and selenate over sulfate but was only effective over a very narrow pH range (Suzuki, 2000). This approach offers the possibility of actively tuning anion selectivity by varying the metal and the cation-anion coordination environment. However, conventional polymeric resins suffer from shrink/swell changes depending on the pH and are not very stable at extreme conditions of pH and temperature (Zagorodni, 2002).
Among the commercialized metal sequestration technologies used today, silica polyamine composites (SPC) are relatively new and have been used very recently in the successful recovery and removal of transition metals, precious metals and mercury from diverse waste streams and mining leaches (Hughes, 2007; Beatty, 1999; U.S. Pat. Nos. 5,695,882; 5,997,748; 6,576,590; 7,008,601). Comparing these polymer modified silica with chelating materials prepared by directly modifying an amino-propyl functionalized silica gel surface, it was found that these materials suffer from degradation in the presence of base, have insufficient mechanical stability, and relatively low capacities due to poor surface coverage.
A need remains for a cost efficient, effective method of removing arsenic from contaminated water sources. The subject means should be highly selective, have a high capacity, and be stable over a long useful life.
All patents, patent applications, provisional patent applications and publications referred to or cited herein, are incorporated by reference in their entirety to the extent they are not inconsistent with the teachings of the specification